The R-T-B based sintered magnet (R represents a rare earth element, T represents one or more elements of iron group elements containing Fe as an essential, and B represents boron), a representative of which is Nd—Fe—B based sintered magnet, is advantageous for miniaturization and high efficiency of the machines using it due to high saturation flux density, and thus can be used in the voice coil motor of the hard disk drive and the like. In recent years, the R-T-B based sintered magnet also has been applicable in various industrial motors, or driving motors of hybrid vehicles, or the like. From the viewpoint of energy conservation and the like, it is desirable that the R-T-B based sintered magnet can be further popularized in these fields. However, when applied in the hybrid vehicles and the like, the R-T-B based sintered magnet will be exposed to a relatively high temperature. Therefore, inhibition of the high temperature demagnetization caused by heat becomes important. For inhibition of the demagnetization under high temperature, a method for sufficiently improving coercivity (Hcj) of the R-T-B based sintered magnet at room temperature is well known as effective.
For example, as a method for improving a coercivity of the Nd—Fe—B based sintered magnet at room temperature, a method in which part of Nd of the compound Nd2Fe14B as the main phase is replaced with heavy rare earth elements such as Dy, Tb and the like is well known. By replacing part of Nd with the heavy rare earth elements, the magnetic anisotropy of crystals is increased, and as a result, the coercivity of the Nd—Fe—B based sintered magnet at room temperature can be sufficiently improved. In addition to the replacement with heavy rare earth elements, addition of elements such as Cu and the like is also effective in improving coercivity at room temperature (Patent Document 1). It is considered that by adding the Cu element, the Cu element forms, e.g., an Nd—Cu liquid phase, at the grain boundary during the production process, and thus the grain boundary becomes smooth, inhibiting nucleation of reverse magnetic domains.
On the other hand, Patent Document 2 and Patent Document 3 have disclosed the technology for enhancing the coercivity by controlling the grain boundary phases which are the microstructure of the R-T-B based sintered magnet. It may be derived from the drawings in these patent documents that, the grain boundary phases as mentioned herein refer to grain boundary phases surrounded by three or more main phase crystal grains, i.e., triple junctions. Patent Document 2 has disclosed a technology for constructing two kinds of grain boundary phases with different Dy concentrations. That is, it has disclosed that by forming a part of grain boundary phases with higher Dy concentration in the triple junctions without increasing the entire Dy concentrations, a high resistance to the reversal of the magnetic domain can be provided. Patent Document 3 has disclosed such a technology in which, three, i.e., first, second and third grain boundary phases which are different in total atomic concentrations of rare earth element is formed in the grain boundary triple junction, the atomic concentration of rare earth element of the third grain boundary phase is lower than that of the other two kinds of grain boundary phases, and in addition, the atomic concentration of the Fe element in the third grain boundary phase is higher than that in the other two grain boundary phases. As a result, the third grain boundary phase containing a high concentration of Fe is formed in the grain boundary phases, which can induce the effect of increasing the coercivity.